Understand solids, liquids, gases and diffusion with particle explanations, three worked examples and 12 original questions with explained answers.
Readnary original study guide
Understand solids, liquids, gases and diffusion with particle explanations, three worked examples and 12 original questions with explained answers.
Original AI-assisted Readnary material. Not independently academically reviewed; answers may contain errors. Check important results against your course materials.
Selected content from 0620 sections 1.1 and 1.2 for exams in 2026-2028. Heating-curve explanations and gas-mass comparisons include Supplement material. This is a focused revision guide, not complete practical or syllabus coverage.
| State | Arrangement and motion | Shape and volume |
|---|---|---|
| Solid | Closely packed; vibrate around fixed positions | Fixed shape and volume |
| Liquid | Close together; move past one another | Takes container shape; fixed volume |
| Gas | Far apart; rapid random motion | Fills available volume |
A particle model connects visible properties to motion and spacing. Solid particles are not motionless: they vibrate around fixed positions. In a liquid, particles remain close but can move past one another. Gas particles are much farther apart. A gas is easy to compress because there is substantial space between its particles; compression does not usually mean that the particles themselves shrink.
Melting changes a solid into a liquid; freezing reverses it. Boiling and evaporation change a liquid into a gas, while condensation reverses that change. In a physical change, the substance keeps its chemical identity. Liquid water and water vapour both contain water molecules. The particles gain or lose energy and their arrangement changes. Do not say that particles 'melt' or 'expand': describe changes in their motion, separation and attractions.
Higher temperature means greater average kinetic energy. During melting or boiling of a pure substance at constant pressure, the temperature can remain constant while energy is supplied. That energy changes the arrangement against attractive forces rather than increasing average kinetic energy. In a fixed-volume container, heating a gas increases pressure because collisions with the walls become more frequent and more forceful. If pressure is instead kept constant with a movable boundary, the gas can expand.
Particles continually move in random directions. When one region has a higher concentration, there is initially a net movement from that region towards a lower-concentration region. Diffusion continues until the distribution is more uniform; individual particles still move after that. Higher temperature generally speeds diffusion. When comparing gases at the same temperature, lower relative molecular mass is associated with faster diffusion. Compare one variable at a time: temperature differences can make a simple mass-only comparison misleading.
A sealed syringe contains air. Its outlet is blocked and the plunger is pushed in slowly. Why can its volume decrease?
A pure liquid is heated steadily at constant atmospheric pressure. Its temperature stays at its boiling point for several minutes. Explain why.
Gases X and Y have relative molecular masses 18 and 44. They start at the same temperature. Which would be expected to diffuse faster?
Try each question before opening its answer. Numerical answers should include your working.
A liquid. Its particles remain close, maintaining roughly the same volume, but can move past one another so the liquid takes the container's shape.
They vibrate around fixed positions. Saying that solid particles are completely stationary would be incorrect.
A gas has much larger spaces between particles. Compression mainly reduces these spaces; particles in a liquid are already close together.
Condensation. The substance transfers energy to its surroundings; its particles form a closer arrangement.
No. Evaporation occurs at a liquid's surface at temperatures below the boiling point. Boiling occurs throughout a liquid at its boiling temperature for the given pressure.
It is used in changing the particle arrangement against attractions rather than increasing average kinetic energy. This is a Supplement-level explanation.
Pressure rises, assuming the container's volume remains fixed. Particles have greater average kinetic energy and collide with the walls more frequently and more forcefully.
State what is held constant, particularly the amount of gas and either pressure or container volume. A flexible container at constant pressure behaves differently from a rigid sealed one.
No. Random motion and collisions of particles allow diffusion. The solute becomes more evenly distributed without requiring bulk stirring.
Particles continue moving randomly. There is no sustained net movement caused by the original concentration difference once the distribution is uniform.
The gas with relative molecular mass 2 is expected to diffuse faster. The comparison concerns molecular mass at the same temperature, not a claim that particles have different sizes in proportion to mass.
The water molecules keep their identity. Energy changes their motion and arrangement as the solid becomes a liquid; melting is not caused by each molecule becoming bigger.
Avoid describing particles as having the same bulk properties as the material: particles are not 'soft', 'liquid' or 'melting'. In heating-curve answers, distinguish energy supplied from temperature change. In diffusion comparisons, check that temperature and the surrounding conditions are comparable.
Chemistry · IGCSE · STUDY GUIDE
Understand solids, liquids, gases and diffusion with particle explanations, three worked examples and 12 original questions with explained answers.
Read the original Readnary guide in PDF view or use the web lesson above. Try the questions before revealing their explained answers.
